Seeking social support and talking about problems are generally thought to be positive adaptive processes. Although some have suggested that there may be downsides to certain support styles, this possibility is understudied. The proposed research offers a more nuanced view on disclosure processes by considering a type of disclosure about problems, "co-rumination," that may simultaneously contribute to both positive and negative adjustment outcomes. Co-rumination will be studied in the friendships of 600 adolescents (300 early adolescents and 300 middle adolescents) who are followed over one year. Co-rumination, which refers to excessively discussing problems, is proposed to have seemingly contradictory relations with adjustment. Co-rumination is hypothesized to be related to positive relationship adjustment (e.g., high-quality friendships) but also to problematic emotional adjustment (e.g., depressive symptoms). Two initial survey studies support the idea that there are social and emotional trade-offs of co-rumination. The proposed research extends the prior survey research by (a) Examining the intrapersonal and interpersonal characteristics that predict co-rumination, (b) Increasing our understanding of the effects of co-rumination on adjustment, including by examining processes that may mediate the association of co-rumination with friendship and emotional adjustment, (c) Examining ethnic differences as well as gender and developmental differences in co-rumination and associated outcomes, (d) Developing an observational assessment and an event-sampling assessment of co-rumination. The multi-method, longitudinal approach described in this application is unusual in the peer relations literature in which much research includes survey measures and a single assessment "snap shot" approach. The proposed research could contribute to a paradigm shift in which researchers more seriously evaluate the costs as well as the benefits of particular support styles. In terms of the value to public health, some youth at risk for emotional problems may be overlooked because they are embedded in a social network. Understanding co-rumination may be important for anticipating the emotional risks for youth who have close relationships and for developing prevention and treatment programs to promote their mental health.